1. Field of the Invention
This invention relates to optically clear organosiloxane compositions. More particularly, this invention relates to curable organosiloxane compositions capable of cohesively bonding layers of glass and/or organic polymers. The cured compositions are sufficiently flexible to compensate for differences in the coefficients of thermal expansion of the materials being bonded.
2. Description of the Prior Art
Many methods have been proposed for imparting adhesion to organosiloxane compositions that cure by the addition reaction of lower alkenyl radicals with silicon-bonded hydrogen atoms in the presence of a platinum catalyst. Some of these methods involve addition of various types of adhesion promoters to the compositions.
Addition of a silane containing both alkenyl radicals and silicon-bonded alkoxy groups or a partial hydrolysis product of the silane to a curable organosiloxane composition is taught in U.S. Pat. No. 4,196,273, which issued to Imai and Tanaka on Apr. 1, 1980, U.S. Pat. No. 4,329,273, which issued to Hardman et al on May 11, 1982, and in Japanese Patent Publication No. 76/28309.
The use as adhesion promoters of silanes containing both silicon-bonded hydrogen atoms and silicon-bonded alkoxy groups and partial hydrolysis products of these silanes is described in Japanese Patent Publication No. 82/5836.
Curable compositions wherein the adhesion promoter is a linear or cyclic polyorganosiloxane wherein each molecule contains substituents selected from lower alkenyl. silicon-bonded hydrogen. silicon-bonded alkoxy, epoxy and ester groups are described in U.S. Pat. No. 3,699,072, which issued to Clark and Hayes on Oct. 17, 1972, U.S. Pat. No. 4,082,726, which issued to Mine et al on Apr. 4, 1978, Japanese patent publication No. 80/39258, and Japanese published application (Kokai) Nos. 54/58755 and 54/58756.
The use of a lower enoxysilane or a partial hydrolyzate thereof as an adhesion promoter is disclosed in U.S. Pat. No. 3,892,707, which issued to Itoh and Harada on July 1, 1975.
Each of the aforementioned prior art adhesion promoters has disadvantages associated with its use. The presence of both a lower alkenyl radical and an alkoxy group in a single silane interferes with the addition reaction and slows the cure rate. Siloxanes containing epoxy or ester groups are not completely miscible the principal siloxane ingredient, and the composition becomes cloudy following the addition. In addition, trimethoxysilane is an extremely toxic material.
Japanese Patent Publication No. 58/101177, which issued on June 16, 1983 discloses improving the moisture resistance and adhesive strength of an adhesive sheet that is placed over a sheet of window glass. The glass is coated with an aqueous solution containing a surfactant and a silane containing a methoxy, ethoxy or silanol group and a vinyl, epoxy, methacryl or mercapto group. The adhesive sheet can be a polyester film that is adhered by means of acrylic ester type adhesives.
U.S. Pat. No. 4,490,500, which issued to Smith on Sept. 9, 1982 discloses compositions suitable for encapsulating electronic components. The compositions contain an organopolysiloxane with terminal silanol groups, an organosiloxane copolymer containing triorganosiloxy and SiO.sub.2 groups, and a prereacted catalyst mixture containing an alkyl silicate or partial hydrolysis product and a stannic carboxylate.
While many prior art additives will enable organosiloxane compositions curable by a hydrosilylation reaction to cohesively bond to a variety of organic and inorganic substrates, the compositions are not suitable for bonding substrates having widely differing coefficients of thermal expansion. The reason for this is that the cured organosiloxane material is not sufficiently flexible to absorb the stresses originating from the different rates at which the bonded substrates shrink or expand with changes in environmental temperature. If not absorbed or dissipated in some manner, these stresses can result in failure of the adhesive bond between the substrates.
An objective of this invention is to provide optically clear organosiloxane compositions capable of cohesively bonding to glass and a variety of organic polymers while exhibiting sufficient flexibility to absorb the stresses resulting from differences in the coefficients of thermal expansion of the materials being bonded.